1. Field of the Invention
The present invention deals with the preparation of certain novel dialkylmagnesiums, commonly designated as magnesium alkyls, and methods for making such reagents and compositions containing said reagents.
Organomagnesium halides, which can be represented by the formula RMgX, where X is halogen, commonly known as Grignard reagents, have wide utility in chemical reactions. By comparison with Grignard reagents, dialkylmagnesium reagents have found less widespread use although such latter compounds commonly undergo reactions analogous or similar to those of Grignard reagents. Primary reasons for the use heretofore of the Grignard reagents over dialkylmagnesiums lies in the fact that, previously, there has been no easy or satisfactory method by which large amounts of dialkylmagnesiums could be prepared, and in certain deficiencies of the dialkylmagnesiums. The present invention involves new techniques for the preparation of novel dialkylmagnesiums relatively simply, in large quantities and free from instability disadvantages characterizing prior dialkylmagnesiums, and makes feasible their effective and economical use for purposes now served by Grignard reagents and by other organometallic compounds, particularly alkyllithiums. While both Grignard reagents and alkyllithiums are presently commercially available, our invention makes possible the production of dialkylmagnesiums at a cost comparable to or less, commercially, than either the Grignard reagents or alkyllithiums if the cost is calculated per organic group (R) and includes both ingredient costs and shipping costs. Furthermore, in certain instances, the dialkylmagnesiums are superior for chemical reasons. Relative to alkyllithiums, a particularly important advantage of the dialkylmagnesiums of the present invention is their greater thermal stability and the lesser pyrophoric character of their solutions and, in this connection, it may be pointed out that stable concentrated solutions of dialkylmagnesiums in saturated acyclic liquid hydrocarbons are not known actually to have been previously prepared. In addition to the values of such latter solutions based on their convenience as easily handled sources of dialkylmagnesiums for use in carrying out known types of reactions, said acyclic liquid hydrocarbon solutions are particularly valuable insofar as they afford dialkylmagnesiums for one-step chemical reactions not attainable with dialkylmagnesiums in othe solvents. The novel dialkylmagnesiums of the present invention constitute a new catalyst system for multi-step reactions such as oligomerizations and polymerizations; and, also, they may be used as basic materials from which other catalyst systems may conveniently be prepared. Typical polymerization processes for which these novel dialkylmagnesiums are very useful are stereo-specific polymerizations of .alpha.-olefins as, for example, ethylene and propylene, and conjugated dienes as, for example, 1,3-butadiene and isoprene. No Lewis base is employed in such processes, the polymerization being run in a liquid acyclic hydrocarbon solvent. such stereo-specific polymerizations of isoprene, for example, yield synthetic rubbers very much like natural Hevea rubber, these rubbers having almost exclusively a cis-1,4- mode of enchainment.
2. Description of Prior Art
U.S. Pat. Nos. 3,646,23l; 3,742,077; 3,755,478 and 3,766,280 are illustrative of disclosures of heretofore known procedures for the preparation of liquid hydrocarbon-soluble diorganomagnesiums.
Thus, for instance, as shown, in U.S. Pat. No. 3,742,077, it is already known, among other things, to prepare hydrocarbon-soluble complexes of hydrocarbon-insoluble primary, linear dialkylmagnesiums, as as di-n-butylmagnesium, with hydrocarbon-soluble secondary branched dialkylmagnesiums, the initial linear dialkylmagnesium compound being prepared directly in a liquid hydrocarbon-solvent such as methylcyclohexane, hexane, benzene or toluene by reacting magnesium metal in said solvent with n-butyl chloride, followed by reaction with a secondary or tertiary alkyllithium to react with byproduct MgCl.sub.2 formed in the first step. The resulting solution, after separation of solids, contains a complex of (a) a primary dialkylmagnesium with (b) a secondary or tertiary dialkylmagnesium. Illustrative complexes disclosed in said patent are those of di-n-amylmagnesium with di-sec-butylmagnesium; di-n-butylmagnesium with di-tert-butylmagnesium; and di-n-butylmagnesium with di-sec-butylmagnesium. These complexes of said patent are also disclosed to be produced by reacting an alkyllithium in sufficient quantity to both react with all of the byproduct MgCl.sub.2 and also to form a complex with the so-prepared dialkylmagnesium compounds or, alternatively, to form complexes of the dialkylmagnesium compounds with alkylsodium or alkylpotassium compounds. The complexes are disclosed to be formed and to be soluble in aliphatic and cycloaliphatic solvents such as heptane, hexane, octane, cyclohexane and methylcyclohexane, but particularly desirably in aromatic hydrocarbons such as benzene, toluene, and xylenes, and compatible mixtures of two or more of the aforesaid solvents. The complex compositions of said patent, in the form of their hydrocarbon solutions, are stated to possess a high degree of stability in comparison with the stability of the uncomplexed solid reagents, such as alkylsodiums and alkylpotassiums.
When pure, dry metallic magnesium is treated with an alkyl halide, in a liquid purely hydrocarbon solvent such as heptane, cyclohexane, or toluene, it is found that only certain alkyl halides, for example, those of the normal or unbranched variety, illustrative of which is n-butyl chloride, react to produce the desired dialkylmagnesiums. The resulting products are, generally, viscous mixtures of partially solubilized dialkylmagnesiums accompanied by a precipitate of byproduct magnesium halide. This mixture of products can be converted into a fluid, halide-free solution of dialkylmagnesiums by interaction with an alkyllithium compound with branching at C.sub.1 in like hydrocarbon solvents.
The set of reactions describing this heretofore known process is as follows: EQU 2R'X + 2Mg .sup.H.C. R'.sub.2 Mg + MgX.sub.2 ( 1) EQU r'.sub.2 mg + MgX.sub.2 + 2RLi .sup.H.C. R.sub.2 Mg .multidot. R'.sub.2 Mg + 2LiX (2)
(where R and R' are alkyl, X is halogen such as chlorine or bromine, and H.C. is a liquid hydrocarbon solvent).
The resulting hydrocarbon solutions of R.sub.2 Mg.multidot.R.sub.2 'Mg, where R' may be n-butyl- or n-amyl, and R is a sec-butyl- or tert-butyl group, possess only a limited solubility in liquid acyclic hydrocarbon solvents, such as n-hexane, n-heptane, or isooctane, generally of the order of 0.5 equivalents (0.25M) per liter of solution, or less, in sharp contradistinction to those dialkylmagnesiums containing both R and R' groups of a branched (at the alphacarbon atom) nature, exemplified, for example, by di-sec-butyl-magnesium or di-tert-butylmagnesium, which are highly soluble, of the order of 2 or 3 moles per liter of solution. However, since such symmetrical, branched dialkylmagnesiums cannot satisfactorily be produced directly from magnesium metal, but, generally speaking, must be less economically produced from activated magnesium halides and more expensive alkyllithiums, an improvement in the solubility in liquid acyclic hydrocarbon solvents of mixed unbranched and branched dialkylmagnesium complexes represents a much desired goal.
Employing the heretofore known techniques described above for their preparation, some dialkylmagnesiums have been found to be insoluble in liquid hydrocarbons, particularly those dialkylmagnesiums with alkyl groups having no branching at the alpha carbon atoms. These insoluble dialkylmagnesiums are represented hereafter as R.sub.2 'Mg or R.sub.2 "Mg (where R' and R" = methyl, n-butyl and isobutyl, for example). Hydrocarbon solutions of these unsolvated dialkylmagnesiums can be produced by complexation with each other.
Based upon our discoveries, several variant procedures for the preparation of acyclic liquid hydrocarbon solutions of complex dialkylmagnesiums have been evolved.